One method of separating a component or group of components from a mixture is selective adsorption on a solid adsorbent. One example of an adsorption process involves a continuous process in which feed and products enter and leave the adsorbent bed at substantially constant composition. The process simulates the countercurrent flow of a liquid feed over a solid bed of adsorbent without physically moving the solid by moving the injection and withdrawal points along the bed. As the concentration profile moves down the column, the injection and withdrawal points also move. The adsorbent-desorbent combination depends on the materials being separated.
FIG. 1 illustrates an example of an adsorption process. Separation occurs in the adsorbent chamber 10. The adsorbent chamber 10 is separated into a number of beds each of which has a flow distributor connected to the rotary valve 15. The flow distributors inject or withdraw liquid from the adsorbent chamber, or redistribute liquid around the chamber. There are four major streams distributed to and from the adsorbent chamber 10 by the rotary valve 15. The feed inlet stream 20 includes a raw mixture of all of the feed components. A dilute extract out stream 25 includes a selectively adsorbed component or components diluted with desorbent. The dilute raffinate out stream 30 includes rejected components diluted with desorbent. The desorbent in stream 35 is the recycled desorbent separated from the extract and raffinate. Only four of the bed lines are carrying streams into or out of the adsorbent chamber 10 at any given time.
A pumparound pump 40 circulates process liquid from the adsorbent bed at the bottom of the adsorbent chamber 10 to the bed at the top. The concentration profile in the adsorbent chamber 10 moves down past the last bed, through the pumparound pump 40 and up to the top. The actual liquid flow rate through the zones is different because the rate of injection and withdrawal of the streams is different. The overall liquid circulation is controlled by the pumparound pump 40 and a flow control valve (not shown).
The dilute extract stream 45 from the rotary valve 15 is sent to an extract column 50 where an extract stream 55 is separated from a desorbent stream 60. The extract stream 55 is then recovered. The desorbent stream 60 is recycled to the rotary valve 15 for use in the process. The dilute raffinate stream 70 is sent to a raffinate column 75 where a raffinate stream 80 is separated from a desorbent stream 85. The desorbent stream 85 is combined with the desorbent stream 60 and recycled to the process. The raffinate stream 80 is removed. A feed stream 90 is sent to the rotary valve 15 for use in the process.
The rotary valve and its operation are described in U.S. Pat. Nos. 3,040,777 and 3,422,848, for example, which are incorporated herein by reference. The rotary valve includes two main parts: a moving rotor plate and a stationary track plate. The rotor plate sits on top of the track plate, and both are positioned in a pressure containing vessel, commonly called a dome. An elastomer seal sheet is secured to the bottom of the rotor plate between the tracks. In normal operation, the rotor plate is hydraulically seated on the track plate by introducing desorbent flow, or dome sealant, into the liquid filled dome. The dome pressure is maintained at a constant pressure during the entire rotary valve cycle, and that constant pressure is always higher than the highest pressure in the tracks below the rotor plate. The dome set pressure is based on the highest seating pressure needed for all of the positions of the rotor plate. Over time, the movement of the rotor plate causes the seal sheet to wear, which can lead to loss of performance and equipment damage.